Hi Salla. Really sorry to hear about this, but try not to worry. Easier said than done, I know.
I had ovarian cysts when I was younger, about 19/20 I think. It was detected only when I had my first child, on the ultrasound scan.
I had had these sharp pains for a little while and, as daffy above, did collapse on a couple of occasions.
They kept an eye on them and they didn't grow but they just disappeared by themselves, without the need for any surgery.
A girl I used to work with also had an ovarian cyst, which although I don't think was 20cm, it was still fairly large, and had it removed and had no further complications or problems :-)